Like other metals, aluminum and alloys destroyed by repeated loads at stress levels significantly lower, than for static loads. This type of fracture, which is the formation of cracks under the influence of alternating voltage, It is known as fatigue failure.
These alternating voltages in practice may be caused by alternating loads, vibration or repeated temperature changes. Direction, wherein the fatigue crack propagates, It is always perpendicular to the line voltage feedback, cause cracks. When a crack grows, voltage in the remainder of the cross-section increases and accordingly increases and crack growth rate.
Finally, stage reached, when the remaining part of the cross section can no longer withstand the applied loads and the sample is destroyed. Fatigue cracks are very difficult to notice, since in contrast to the test specimen at break no visible thinning of the specimen at the place of destruction.
Cycle fatigue loading
To describe the state of stress, leading to fatigue, применяют следующих четыре основных параметра:
1) минимальное напряжение цикла, pmin;
2) максимальное напряжение цикла , pMax;
3) среднее напряжение цикла, pm = (σmin + pMax)/2;
4) амплитуда напряжения цикла, pa = (σmin – pMax)/2.
Voltage cycle is fully defined, if you know any two of these four parameter.
Влияние среднего напряжения цикла на усталостную прочность очень значительно: наложение на симметричный цикл дополнительных растягивающих напряжений снижает усталостную прочность, imposition of compressive stress enhances ee.
Fatigue resistance is usually greatly reduced by the presence of local stress concentrators, such as acute groove, sharp change even thickness profile or rough machining marks and scratches.
Particular influence on fatigue strength, these surface defects and stress concentrators have, if they are located transversely to the direction of stress application. They may have a much greater influence on the fatigue strength, than those differences, which may be from different alloys or various treatments. Therefore always key considerations for the fatigue strength of structural members are of regular shape and surface quality.
Weller curve for aluminum
The usual procedure is to test the fatigue test, a certain number of samples at the different levels of voltage amplitude. When applying the test results on a graph voltage-number of cycles to failure is obtained fatigue curve (drawing 1). It is also often referred to as curve Weller and S-N diagram.
In steels fatigue curve has a flat portion, defining endurance strength. At voltages below this limit fatigue never collapse. Most of the aluminum alloys, as well as other non-ferrous metal, не имеют такого предела (исключением могут быть только алюминиево-магниевые 5xxx series alloys), and the curve is sloping indefinitely. Therefore, for most aluminum alloys are always a result of cyclic loading will be destruction, although the range of values greater durability slope of the curves becomes small.
Fatigue of aluminum
To denote voltage, при котором происходит разрушение алюминиевых сплавов (и большинства других цветных сплавов) используют термин fatigue strength at a predetermined durability, which is denoted by σN. The term fatigue strength determines the voltage amplitude, in which the destruction occurs after a specified number of cycles. The term also always corresponds to the fatigue limit of infinite durability.
of tensile strength data, yield strength and fatigue strength of some aluminum alloys in the various states shown in comparison in Figure 2.
Source: TALAT 1501